1. Field of the Invention
The invention relates to an extruder, in particular for processing polymer melts, with an extrusion space which is arranged in an extruder barrel and through which at least one extruder screw shank extends in such a way that it can be rotated by a motor, said screw shank having on it at least one section in the form of a circumferential bulge, the extrusion space having at least one barrel section which surrounds in an essentially equidistant manner the at least one extruder screw shank in the area of the at least one circumferential bulge to form an annular gap as a restricted flow zone (blister), in particular to set the pressure of the melt.
2. Description of the Related Art
An extruder of this type is disclosed by U.S. Pat. No. 4,332,481.
Such a restricted flow zone formed as an encircling gap, for example as an annular gap, in the extrusion space is also referred to as a blister and serves in particular for setting the pressure of the melt processed in the extruder. Downstream of this restricted flow zone, entraining agents may be added for example, being incorporated into the expanding polymer melt and serving as stripping agents. The effect of the blister is strongly dependent on the clearance in the annular gap between the cylindrical part of the extruder screw and the inside surface of the cylindrical extrusion space. In practice, the clearance is often of the order of approximately 3 mm. The optimum clearance is dependent less on the size of the respective extruder than on the properties of the material to be processed (temperature, viscosity). According to the prior art, changing of the clearance often involves a corresponding change of the screw shank or the cylindrical part of the screw shank in the region of the restricted flow zone. The processing of different materials therefore frequently requires the screw shank to be changed in order that best possible processing conditions can be set. In the case of non-steady-state processes during processing, the optimum size of the annular gap of the blister changes. Until now there has been no possible way of optimizing the clearance in such a situation. In particular during start-up operations, in which the extruder has not yet reached a constant operating temperature, changes in the effective clearance of the blister may be brought about by different degrees of thermal expansion. It has so far also not been possible to deal with this in an optimum way.
DE 198 31 540 A1 discloses a device for partially changing a closed flow channel cross section, the inner contour of which is formed by a displacement element, for example of a cylindrical shape. The flow channel is preferably part of an extrusion die for producing a tube from flowable compositions. The device has external adjusting elements in the form of pins which can be adjusted radially from the outside inward and at least one region of a flow channel wall which comprises a sleeve with a wall thickness of at most 1 mm, the sleeve being locally deformable inward by means of the adjustable pins. The displacement element arranged in the flow channel may likewise be provided with a sleeve of a small wall thickness, which is locally deformable outward by means of supporting elements that are axially displaceable on conical supporting surfaces. In this way, the flow channel cross section can be reduced in a specifically selective manner in the region of the thin flow channel wall. A disadvantage of this known device is that the adjustable pins and displaceable supporting elements do not allow uniform deformation over the cross section of the flow channel, since the introduction of the deforming forces can only ever take place in a punctiform or locally limited way. The greater the number of elements used for the deformation, the more uniformly the deformation can be set, but the greater the actuating effort for these elements as well. Another disadvantage is that the thin walls of the sleeves are unavoidably deformed by the melt pressure prevailing in the flow channel counter to the force of the elements used for the adjustment. Consequently, it is impossible for the device to be operationally used under geometrical conditions of the entirely unloaded state (zero position).
Further possibilities emerge from JP-60002329, where regions of an extruder barrel which can be elastically deformed by adjusting devices and interact with screw segments to produce an adjustable restricting gap are provided, or from U.S. Pat. No. 3,122,789, which shows a membrane which can be elastically deformed by a pressure fluid to adjust the flow channel height in a slot die.
It is an object of the invention to develop an extruder of the type stated at the beginning to the extent that a largely optimum mode of operation in the region of the blister is possible even if different materials are to be processed or the material properties change, without any major modification work being required for the extruder. The effort for the adjustment of the blister is to be as little as possible.
This object is achieved in the case of an extruder of the generic type by the clearance of the encircling gap at the blister being able to be changed in a specifically selective manner by externally adjustable force, with elastic deformation of the surface of the extrusion space and/or of the screw shank. For this purpose, the invention provides a hydraulic pressure unit which supplies the force for the surface deformation in the region of the restricted flow zone either on the inside surface of the extrusion space or on the circumferential bulge of the at least one screw shank or else on both, a hydraulic pressure chamber being provided in the region of the restricted flow zone in the wall of the barrel section where the blister is to be arranged and/or in the circumferential bulge of the at least one screw shank. The pressure chamber has on the side respectively facing the extrusion space a considerably smaller wall thickness in comparison with the wall thickness of the extrusion space. This wall is referred to hereafter as a membrane wall because of its small thickness.
The pressure chamber is expediently provided respectively in said barrel section of the extruder barrel and not on the screw shank. The reason for this is primarily to be seen in the fact that, because of the associated sealing problems, it is more difficult to provide a supply of hydraulic pressure medium to the pressure chamber on the screw shank than on the extruder barrel, which unlike the screw shank is completely immovable.
The invention can be realized particularly advantageously in the case of a single-screw extruder, which has a purely cylindrical extrusion space, with the result that the encircling gap at the blister is formed as an annular gap. The hydraulic pressure in the pressure chamber allows the membrane wall to deform to the respectively desired dimensions in a geometrically very uniform shape (circular shape). The desired settings can be controlled extremely quickly and easily, since all that matters is the difference in pressure between the hydraulic pressure and the pressure in the extrusion space.
However, the invention can also be applied very advantageously in the case of twin-screw extruders. In these extruders, the extrusion space is formed by two longitudinal bores which overlap partially in cross section, at least over part of the axial length of the extruder barrel, and because of their shape are also referred to as a xe2x80x9cspectacle-borexe2x80x9d. Arranged in this spectacle-bore are two co-rotatable or counter-rotatable motor-driven screw shanks which lie parallel to each other and each have at least one circumferential bulge. The circumferential bulges of the two extruder screw shanks in this case respectively lie directly next to each other. If the pressure chamber is not arranged respectively on the two extruder screw shanks but is provided on the inside surface of the barrel of the extrusion space, that is to say in the spectacle-bore, it is recommendable not to form the pressure chamber as a completely encircling chamber but to divide it into two separate chambers which are arranged mirror-symmetrically in relation to each other and, in the cross section of the extruder barrel, extend only in each case over by far the greater part of the circumferential bulges of the longitudinal bores (spectacle-bore), with the two interstitial regions being cut away. Consequently, in the transitional region between the two longitudinal bores, the two chambers are not contiguous.
A particularly advantageous embodiment of the invention in the case of a twin-screw extruder is obtained by providing a special barrel section which, instead of a spectacle-bore, has two corresponding fully cylindrical, i.e. not overlapping, longitudinal bores for the two screw shanks, these longitudinal bores of course having a smaller diameter than the respective partially cylindrical overlapping bores of the spectacle-bore. The diameter of the fully cylindrical bore preferably corresponds in each case to the root diameter of the respective screw shank, that is to say is only slightly larger than the latter. In this embodiment, in this special barrel section there is consequently a material bridge between the two longitudinal bores at the location corresponding to the interstitial region of the spectacle-bore. Respectively arranged in the surface of the longitudinal bore in this barrel section is a hydraulic pressure chamber, which is designed such that it runs all the way around in the same way as in the case of the embodiment for a single-screw extruder. This solution permits a completely uniform setting of the respective annular gap over the entire circumference. The pressure chambers of the two longitudinal bores can be partitioned off fully from each other, but may also be directly connected to each other in conducting terms.
Since the parts of the screw shanks provided with flights have a greater outside diameter than the fully cylindrical longitudinal bores of the special barrel section, they cannot be pushed through the latter during assembly. Therefore, either the barrel section must be of a divided design, which would correspondingly also require division of the hydraulic pressure chamber in the longitudinal bore, or the screw shank is respectively divided into axial portions which can be coupled together, for example by means of plug-in connections. It is also possible to use screw elements with a central through-bore, which are pushed into the extruder barrel from both ends of the latter and are subsequently securely connected to each other by means of a tension rod passed through the through-bore.
It is recommendable to arrange the pressure chamber of the blister in a separate, essentially annular part of the barrel which has only a short axial length, consequently the length of which is less than the inside diameter of the extrusion space. This annular barrel part is expediently connected to the other parts of the extruder barrel by a flange connection.
It is conducive to aspects of production engineering if the pressure chamber is made as a welded structure. In this case, the welds should advantageously be provided outside the membrane wall in thicker-walled regions, in order as far as possible not to allow the membrane wall itself to get into the zone of thermal influence of the weld, which could change its material properties unfavorably.
The membrane wall should be dimensionally designed for a difference in pressure of at least 100 bar, preferably of at least 150 bar. In any case, it must be ensured that the forces on the membrane induced by the hydraulic pressure applied in the pressure chamber lead only to elastic deformations, but not to plastic deformations. Furthermore, the loading should lie below the critical buckling load, in order that there is no wave-like deformation in the circumferential direction on the membrane wall which is arranged on the inside surface of the extrusion space. This is because it would mean that the effective clearance of the blister varies in the circumferential direction. In practice, the expedient thickness of the membrane wall lies in a range from approximately 0.5 to 3 mm, preferably in the range from 1 to 2 mm. However, these figures are not to be regarded as restrictive. In principle, the degree of deformation not only depends on the thickness of the membrane wall and the applied pressure in the pressure chamber but is of course also strongly influenced by the axial length of the membrane wall, i.e. by the length of the extent of the pressure chamber in the direction of the axis of the screw shank. The longer the membrane wall, the greater the deformation under the same pressure.
The invention can be used with particular advantage on an extruder in which a multi-screw extruder section is arranged downstream of the blister in the direction of material flow. This multi-screw extruder section may be designed in particular as a planetary-gear extruder section.
By providing an extruder with, a blister in the way according to the invention, allowing the effective clearance of the annular gap to be changed unproblematically at any time in a simple, specifically selective manner by elastic deformation through a force which can be adjusted from outside, it is possible without changing the screw shank to process different materials under optimum process conditions, provided that the remaining screw shank elements are suitable in principle for the respective material. In the case of non-steady-state processing conditions, it is possible in the course of operation to make an optimum choice of the clearance of a blister and realize it. In the case of a blister according to the invention, with a hydraulic pressure chamber and membrane wall, this is possible by simply presetting the hydraulic pressure in the pressure chamber or the difference in pressure with respect to the pressure in the extrusion space.
The invention is explained in more detail below on the basis of exemplary embodiments in the drawing.